In a method for fabricating a semiconductor device, it is necessary to measure thickness of insulating films of silicon oxide, etc. and polysilicon films formed on a semiconductor substrate. In a conventional method, the thicknesses are measured by radiating a light beam on the films. For instance, an insulating film is provided on a silicon substrate, and a film, a thickness of which is to be measured, is provided to contact via an aperture of the insulating film with the silicon substrate.
In measuring the thickness of the film, a light beam is radiated on the film, and strength-wavelength distribution of a light beam reflected from the film is measured. In this manner, the thickness of the film is measured on the basis of the wavelength distribution of a reflected light beam.
In the conventional method for measuring a thickness of a film in the fabrication of a semiconductor device, however, there is a disadvantage in that it is difficult to analyze the wavelength distribution of a reflected light beam, so that the reliability of a semiconductor device thus fabricated is hard to be enhanced.
Further, if a semiconductor device is fabricated on a SOI substrate as described on pages 51 and 52 of the paper in the '91 VLSI symposium, a single crystal silicon layer which is a part of the SOI substrate tends to deviate in thickness. Therefore, a thickness of the single crystal silicon layer for the SOI substrate must be measured precisely.